Bacteria and Quorum Sensing

Before talking about anything else in this blog, I would like to explain one of the pathogens that causes many diseases. The purpose is to help you better understand my future posts because some of them might be related to pathogens.

So today, I would like to talk about bacteria and one of its fundamental mechanisms. Bacteria is a single-celled microscopic organism with few genes. They live in and on other organisms such as human and grow by consuming nutrients from environment. These bacteria have symbiotic relationship with the host; mutualism, commensalism, or parasitism. The size of a bacteria is so small that individual bacteria cannot affect the host by itself but when many bacteria act together to achieve the same goal simultaneously, the effect cannot be overlooked.

bacteria

Bacteria work and communicate simultaneously by recognizing other neighboring bacteria community using chemical signal molecules, autoinducers. They produce, release, detect, and recognize autoinducers for chemical communication. Quorum sensing is a mechanism which allows bacteria to monitor the environment for other bacteria and to alter behavior on a population-wide scale in response to changes in the number present in a community by detecting autoinducers.

quorumSensing

Autoinducer concentration increases as bacterial cell population density increases. These chemical molecules accumulate and after reaching a threshold level, bacteria detect autoinducers and alter gene expression. To simply explain the mechanism, a bacterial cell has signal producing protein and it produces autoinducers intracellulary then released outside of the cell. Also, bacterial cell has signal receptor protein for autoinducer. When autoinducers’ concentration passes a threshold, autoinducer binds to receptor protein triggering signal transduction cascade that result in population-wide changes in cells’ behavior. However, when a single bacteria performs the mechanism it is futile since there are no other bacteria to receive the signal and concentration of autoinducers cannot reach the threshold.

Pathogenic bacteria use quorum sensing to promote virulence factor expression to cause disease. For example, “S. aureus” is a very dangerous opportunistic pathogen which has been incresingly associated with antibiotic resistance. One of the factors that contribute to “S. aureus” virulence is its peptide-based quorum sensing system and regulation of biofilm formation, a central factor in S. aureus virulence.The accessory gene regulator (agr) system is intricately involved in the regulation of virulence genes P2 and P3 which produce RNA II and RNA III. Transcription from P3 leads to production of the effector molecule of the agr system, RNA III. RNA III functions to increase the production of capsule, toxins, and proteases. One of virulence factors regulated by agr contains genes involved in production of exoproteins associated with invasion and toxin production. Thus, activation of agr system in “S. aureus” turns the bacterium from commensal to aggressive and invasive pathogen.

As quorum sensing controls virulence, it has been considered an attractive target for the development of new antobiotics. One way is making disease-specific anti quorum sensing molecule for intra-specific communication. It looks similar with autoinducer and binds to receptor but does not initiate signal transduction cascade, thus, inhibiting virulence. The other way focuses on inter-species communication. It works similarly with the disease-specific anti quorum sensing molecule but the molecule binds to inter-species receptor so the cell cannot recognize other surrounding bacteria. It is targetted to be used as broad spectrum antibiotics that work against all bacteria.

 

References:

http://www.annualreviews.org/doi/pdf/10.1146/annurev.cellbio.21.012704.131001
http://www.annualreviews.org/doi/full/10.1146/annurev-genet-102108-134304
http://mic.sgmjournals.org/content/156/8/2271.full

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9 comments on “Bacteria and Quorum Sensing

  1. joekuhn93 says:

    Sangmin402,

    This blog is particularly interesting. I did not know that bacteria have the capability of “sensing” or chemically interacting with other neighboring bacteria around them. Even though they are single-celled, they seem like they are a lot more sophisticated. What interested me the most was that these bacteria had to reach a threshold before they were capable of causing disease or other sicknesses. I like thinking that individual or small micro-groups of these bacteria are “powerless” to my body, but when there are a dangerous amount of them, that’s when things get scary.

    I have one question regarding the antibiotics of treating these bacteria. Are there particular forms of antibiotics that are used to work against all types of bacteria? If so, that would be particularly helpful for treating all forms of sicknesses relating to bacterial infection.

    -joekuhn93

    • sangmin402 says:

      I learned in my microbiology class that there are several types of bacteria that share similar characteristics base on their plasma membrane. The plasma membrane of bacteria is composed of different molecules that are used to differentiate among the different types. If we are able to destroy the plasma membrane of a bacteria, the bacteria will eventually die because the intracellular condition and extracellular condition are very different One example is the gram-positive bacteria and gram-negative bacteria. The composition of their plasma membrane differs from each other. So to target either gram-positive or gram-negative bacteria, scientists use a specific antiobiotic that will attack a specific molecule in their plasma membrane. I hope this example helps.

  2. jaclynckrogh says:

    We rely heavily on antibiotics today, but we keep hearing that bacteria mutates and then the antibiotic is no longer useful. I wonder if with this new kind of antibiotics the bacteria will be able to become immune or if it will always be effective. It is interesting to think about the different ways to stop a disease in its tracks.

    • sangmin402 says:

      I don’t really have a clear answer to you question. But from my knowledge, any kind of mutation could lead to overcomming this new kind of antiobotic. Since bacterias mutate at a constant high rate, there are probabilities the immunity could occur.

  3. pfrowan16 says:

    To think that something we rely on as heavily as antibiotics can potentially mutate into something that seems all that much more harmful is somewhat frightening. In a way its also very interesting thinking about how everything is capable of evolving to modify itself, even something as small as bacteria. Regardless, I have faith in the scientists and research labs across the globe will find an answer to this problem.

    • sangmin402 says:

      I agree. The antiobody and bacteria’s mutation relationship wil occur for ever. Each time scientist discover new antiobodies, there are new strains of bacteria forming that are immune. I guess this pattern will continue to exist as scientist discover new antiobodies.

  4. The fact that you used the word quorum sensing made me click. I was impressed. Thank you for your post (from a former chemotaxis grad student).

  5. […] gets better. Maybe you have heard of quorum sensing (check here). Bacteria produce and release a chemical signal that is recognized by other cells and gives them […]

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